You can use the Joomla settings to enforce https redirect, but if in some case this does not work properly you can use the following code in your .htaccess file.

To force all of the web traffic (every link in your website) to use HTTPS insert the following lines of code in the .htaccess file:

RewriteEngine On
RewriteCond %{HTTPS} off
RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]

IMPORTANT

If you have some existing code in your .htaccess, make sure that 

RewriteEngine On 

is not repeated twice. In case the line already exists, simply copy the rest of the code without it. Otherwise the https redirect will not work.